Re: [fetch] Header guards (#182)

@phuu I can write a bunch of tests that will pass with a real fetch implementation and fail with a polyfill implemented as you describe. For example, `Object.getPrototypeOf(response.headers) === Headers.prototype`, `response.headers.constructor === Headers`, etc.

It seems better to implement this as the spec says, with a single Headers class and an internal (i.e. underscore-prefixed) guard.

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